Voltage source converters are of interest to use in a number of different power transmission environments. They may for instance be used as voltage source converters in direct current power transmission systems such as high voltage direct current (HVDC) and alternating current power transmission systems, such as flexible alternating current transmission system (FACTS). They may also be used as reactive compensation circuits such as Static VAR compensators.
In order to reduce harmonic distortion in the output of power electronic converters, where the output voltages can assume several discrete levels, so called multilevel converters have been proposed. In particular, converters where a number of cascaded converter cells, each comprising a number of switching units and one or two energy storage units in the form of DC capacitor have been proposed. These converters are also known as chain-link converters.
Converter cells in such a converter may for instance be of the half-bridge, full-bridge or clamped double cell type.
A half-bridge cell provides a unipolar voltage contribution to the converter and offers the simplest structure of the chain link converter. This type is described by Marquardt, ‘New Concept for high voltage-Modular multilevel converter’, IEEE 2004 and A. Lesnicar, R. Marquardt, “A new modular voltage source inverter topology”, EPE 2003. This module is effective in that the number of components is low.
However, there are a few problems with the half-bridge topology in that the fault current blocking ability in the case of a DC fault, such as a DC pole-to-pole or a DC pole-to-ground fault, is limited and that it is unable to provide bipolar voltage contributions.
One way to address this is through the use of full-bridge cells. This type of cell is for instance described in WO 2011/012174. A converter using full-bridge cells will be able to both block fault currents caused by DC faults and are able to provide bipolar voltage contributions.
However, the use of full-bridge cells doubles the number of components compared with a half-bridge cell.
One way to reduce the number of components is through the use of clamped double cells or clamp-double submodules. These cells have two sections, where each section comprises an energy storage element having a positive and a negative end and a pair of switching units in parallel with the energy storage element. The junction between the switching units of a section furthermore provides a cell connection terminal. A further switching unit connects the negative end of one of the energy storage elements with the positive end of the other energy storage element. There are also two clamping diodes, one between the positive ends of both energy storage elements and one between the negative ends of the two energy storage elements. A description of the cell has also been made in WO 2011/067120. This type of cell is advantageous in that it has fewer components than the full-bridge cell and that it allows fault current limitation. However, the voltage contributions are also unipolar.
The modular multilevel converter is thus a promising topology for high-voltage high-power applications. By the series-connection of cells it can generate high-quality voltage waveforms with low harmonic distortion at low switching frequencies. The cells can thus be seen as low-voltage ac-dc converters with capacitive energy storages. These capacitive energy storages are a driving factor of the size, weight, and cost of the converter. For this reason it is important to ensure that the stored energy in the converter is distributed as evenly among the cells as possible. During nominal operation, large amounts of energy is moved between the arms in the converter.
It would therefore be of interest to obtain a cell requiring a lower number of components in the conduction path than the full-bridge cell, while still having the ability to provide bipolar voltage contributions and fault current blocking.